Hemoglobin molecule is a prototype of an allosteric molecule whose function is governed by its conformation which in turn is regulated by a large number of small molecules such as organic phosphates, CO2, Cl-, etc. (allosteric effectors). This work deals with the study of the influence of conformational changes on enzymatic methemoglobin reduction as well as with several aspects of oxidation. Methemoglobin reductase was purified by standard methods and the reduction of alpha and beta subunits was studied in tetrameric molecule. It was shown that methemoglobin reduction is a biphasic reaction due to the difference in reduction rate for different subunits. Beta-subunits have a higher reduction rate. It was also shown that inositol hexaphosphate (IHP) which shifts the conformation of methemoglobin towards (T) accelerates only the reduction rate of high spin methemoglobins (aquomet and fluromethemoglobin) but not that of low spin (cyanomethemoglobin). It was shown as well that the Beta-93 free-SH groups have a profound effect of the oxidation of hemoglobin by nitrite. Blocking of these groups by iodoacetamide accelerates the oxidation of deoxyhemoglobin as well as abolishes the effect of IHP on oxidation under aerobic conditions. It is concluded: a) That organic phosphates play an important role in methemoglobin reduction (low reduction rate at red cell pH and its maximum stimulation by organic phosphates). b) The Beta-93 free-SH groups participate more directly in hemoglobin oxido-reductive sections and have a protective role against heme iron oxidation.